The invention concerns a process for the production of a multi-layer body having a volume hologram, a master for the production of the volume hologram and a security element having the multi-layer body.
Holograms are used as security elements for protecting security documents such as banknotes, identity cards or passports, security cards or the like to achieve a high level of forgery-proofness. For mass-produced items, surface holograms are frequently used, which on the one hand do not produce an optimum image impression and which on the other hand can be copied by copying the shape of the surface relief.
Volume holograms, also referred to as white light holograms or reflection holograms, are based on light diffraction at what are referred to as the Bragg planes of a transparent layer which has local differences in refractive index and they produce a brilliant image impression. They cannot be copied by shaping of a surface relief.
It will be noted however that simultaneously writing two different items of image information into a volume hologram gives rise to weak-light and/or blurred reproduction in the regions in which the two items of information are in overlapping relationship. That can be avoided if two separate holograms are arranged one over the other. A disadvantage in that respect however is on the one hand the increased thickness—volume holograms are of a thickness which is a multiple of the light wavelength used to produce them—and the high demands in terms of register accuracy of the volume hologram layers.
EP 1 187 728 B1 describes a process which provides that a transmission hologram and a reflection hologram are respectively recorded in a hologram layer and then the two hologram layers are laminated on to each other.
EP 1 217 469 A2 discloses a process of coating a surface relief hologram with a photosensitive layer and producing a volume hologram by means of the contact process.
EP 1 511 636 A1 describes a process in which a volume hologram is produced by an optical copying operation from a master with a surface relief, the surface relief being a hologram.
Such volume holograms are admittedly more forgery-proof than the master hologram, but they are optically no higher in quality than a surface hologram.